fix: dashboard errors — Math.abs, API routing, fetchHistorical guard

Fixes three bugs in dashboard:

1. vbt.html: bare abs(dd) → Math.abs(dd) — fixes ReferenceError
2. server.py: Add /cv/ routing for Next.js quant dashboard
   - Mount _next static assets at /cv/_next (not /cv/ which eats API routes)
   - Add /cv/api/* routes for backtests/historical, detail, recalc, risk
   - Add /cv/ws WebSocket endpoints for live/paper metrics
   - Add /cv/ catchall for Next.js HTML pages
3. dashboard-next/src/lib/api.ts: add res.ok guard to fetchHistorical()
   — prevents SyntaxError when API returns HTML error pages
4. sim/maker.py: guard observe() against zero mid_price

dashboard-next/src/lib/api.ts

@@ -79,6 +79,7 @@ export function usePaperMetrics() {
 
 export async function fetchHistorical(): Promise<Record<string, import("./types").BacktestSummary>> {
   const res = await fetch(`${API_BASE}/backtests/historical`);
+  if (!res.ok) throw new Error(`HTTP ${res.status}`);
   const list: import("./types").BacktestSummary[] = await res.json();
   const byStrat: Record<string, import("./types").BacktestSummary> = {};
   for (const b of list) {